These stressors also wear away at bridges over time, leading to long-term damage. Inspectors, managers, and engineers must look for these signs. It can help them keep existing structures safe and provide them with the information they need to design even more durable and responsive structures in the future. Bridge Masters takes your privacy seriously and will never sell or share your information. The gravity dilemma The most profound force affecting bridges is gravity, which is constantly pulling at them, trying to drag them down to earth.
The answer is pretty much the same no matter the type of structure: Compression a force that pushes or squeezes inward is carefully balanced with tension a force that stretches and pulls outward. This balancing happens by channeling the load the total weight of the bridge structure onto the abutments the supports at either end of the bridge and piers the supports that run under the bridge along its length.
These forces are distributed in a variety of ways on different types of bridges: Beam Bridge A beam bridge has its deck beam in tension and compression. Arch Bridge An arch bridge supports loads by distributing compression across and down the arch.
Suspension Bridge The towers piers of a suspension bridge are in compression and the deck hangs from cables that are in tension. Cable-stayed bridge A cable-stayed bridge is similar to a suspension bridge.
Truss bridge A truss bridge is a variation of a beam structure with enhanced reinforcements. Cantilever bridge A cantilever bridge is one of the simpler forms to understand. Check out these bridges that manage forces in unique ways: The Rolling Bridge, London This sculptural structure is a type of bridge commonly referred to as a curling bridge.
Stressors beyond gravity The complicating factor is that compression and tension on a bridge are constantly shifting because of stressors like: Changing loads It would be easy to build bridges if the loads on them stayed static.
Environmental forces Bridges constantly react to Mother Nature. Environmental sources of stress include: Tides, waves, and water back-ups. Water is one of the most powerful forces on earth. Engineers often insert openings into bridge abutments to allow water to flow through rather than push against them.
Large gusts of wind can cause bridges to sway and twist. Modern ones are lighter and more aerodynamic, allowing wind to pass through them, which prevents them from moving. Seismic forces cause bridge sections to shake and crash into each other, which can make them crumble.
Designers include dampers to absorb vibrations and bumpers to keep sections from banging into each other on bridges in active earthquake zones. Hurricanes and other major storms can have devastating effects on exposed areas of bridges.
Construction teams often install protective equipment around vulnerable sections, such as utility infrastructure. Engineers may categorize stone arch bridges based on the shape of the arch semicircular, segmental, or elliptical. Steel arch bridges, like concrete arch bridges, can be arranged as deck arch bridges or through arch bridges. Steel arch bridges are further categorized based on the design of the arch rib, the presence of hinges, and any bracing. Deck arch bridges sometimes include diagonal bracing in addition to the vertical columns.
In these cases, the bridge is known as a spandrel-braced arch bridge. A bridge can have hinges at the skewbacks bearings only a two-hinged arch bridge.
A steel arch bridge can also have a third hinge at the crown mid-span , making it a three-hinged arch bridge. A suspension bridge consists of a deck that is held underneath main cables that are stretched out over the span from tall towers that rise above the deck. The far ends of the main cable known as backstays are held in place by anchorages. The deck itself is kept stiff in the presence of wind and the loads that pass over the bridge by way of a stiffening system.
The stiffening system can take the form of various simple bridge types. With historic bridges, the stiffening system typically takes the form of a truss pony, through or deck or girder through or deck. A three span suspension bridge, with each of the three spans suspended from a main cable is the most common configuration of a suspension bridge. A rare design of suspension bridge eliminates the use of large anchorages by connecting the bridge to itself: the main cables are fastened directly to the bridge superstructure at the ends of the bridge, forming a self-anchored suspension bridge.
Some suspension bridges only have a single suspension span. In this case, the main cable becomes an unloaded backstay cable that proceeds directly from the top of the tower down to the anchorage. This layout is shown above. The main cable can take the form of a wire cable, or an eyebar chain.
Eyebar chain bridges are rare; most suspension bridges use some form of wire rope for the cable. One of the common types of the basic suspension bridge form is a the cable-stayed bridge. In a cable-stayed bridge, the individual suspender cables directly connect from the tower to the deck, with no catenary main cable involved.
Cable-stayed bridges tend to be more stable than traditional suspension bridges, and thus often do not have large stiffening trusses and girders. Cable-stayed bridges can feature cables configured in a variety of manners. Double Vertical Plane Parallel : If the cables spread out to both sides of the deck from double posts one post for each side of the deck , it is a double vertical plane cable-stayed bridge.
Double Vertical Plane Inclined : In a double vertical plane, the posts can be inclined as well. Single Vertical Plane: If the cables spread out to both sides of the deck from single posts in the center of the deck, it is a single vertical plane cable-stayed bridge. Many of the bridge types described above can be designed alongside mechanical features to allow the bridge to clear a waterway and make way for boats that would otherwise not fit underneath the bridge when closed.
These movable bridges are categorized based on the design of the mechanical features that enable the movement of the bridge. Bascule bridges rotate up to open for boats. They include a counterweight to balance the span and enable relatively small motors to raise the large leaf. If the span consists of two bascule leaves that raise up, the bridge is a double-leaf bascule.
Most highway bascule bridges are double-leaf. Among movable bridge types, bascule bridges display the most variety in mechanical operation. The following bascule designs are adapted from those which appeared in J. Fixed-Trunnion Chicago Type : The counterweight is fixed to the leaf, and the leaf rotates around a fixed trunnion.
Scherzer Rolling Lift: This type of bascule rotates on a track, and has a counterweight fixed to the leaf. Invented in by Albert Scherzer and popularized by his brother William Scherzer.
Strauss Bascule: Joseph Strauss invented his variety of trunnion bascule bridge that is noted for having a separate trunnion for the counterweight. As such, the counterweight is not fixed to the leaf. This overhead counterweight design was also adaptable to a design where the counterweight was hidden below the roadway.
Heel-Trunnion: This variation on the Strauss bascule is noted for its parallelogram shape and typically takes the form seen above. The counterweight is always above the roadway. A movable bridge type, swing bridges are typically classified into two major categories based upon how they rest on the swing pier as noted below.
Most swing bridges are symmetrical center pier but if the pier is not at the center, then the bridge is a bobtail swing bridge and a balancing counterweight may be present at the shorter end.
Rim Bearing: Span bears upon the circular track around the rim. Typically has a dense series of numerous rollers around the rim. Center Bearing: Span bears upon a single point at the center of the swing pier. As such, there may be less rollers around the rim as they only serve to guide the truss when moving, and are not load-bearing.
Some swing bridges are designed such that the swing pier is not in the center of the bridge, with one arm offering a swing span over the waterway, and another shorter arm having a counterweight to keep the bridge balanced.
These are known as bobtail swing bridges. A movable bridge type, vertical lift bridges rise directly up to provide clearance for boats. The two most common forms utilize tall towers that house counterweights that move to keep the span in balance as it is lifted by motors.
These types of vertical lift bridges are categorized based on where these motors are located. The above diagram, created by famous vertical lift bridge engineer J. Waddell shows the many parts of a vertical lift bridge. An arch bridge is the most popular type of bridge which is extensively used by ancient Romans. The arch bridge is usually made up of stone, concrete or steel. As the name itself mirroring that, the bridge is in the shape of an arch.
An arch bridge is a curve shaped bridge where the load on the curve is not directly applied straight down, but instead, loads are carried along the curve of the arch to the end of supports. Meaning that no part of the bridge takes a high amount of pressure.
These supports are also called as abutments. The no. The span length of arch bridges is normally up to m , and the roadway of the bridge lies on the arch structure. Easy to build with the locally available material. Arch bridges are built up with a variety of materials like stone, concrete, steel, etc. They take a long time to build. It requires a massive amount of building materials to build. Truss is a framework consisting of struts inclined members. These bridges are constructed by using trusses which are comprised of many small elements forming triangular trusses.
The span length of truss bridge is in between 50mm. Trusses are very rigid, lightweight and can support heavy loads.
Trusses serve in transferring the load from a single point to the wider area. The weight of the bridge is very less when compared with other types. When the load is applied to the truss bridge, the top edge possess compression and loads are shared among the angled members to supports and then to earth. This type of bridge is easily built in the factory and then framed on site. The piers or supports are comparatively less when compared to the beam bridge, 3. They are strong and rigid and very light on weight possess efficient use of materials.
Requires high skilled professionals to design it. They are more complex than beam bridges in terms of designing. This type of bridges are constructed by suspending the deck slab using suspension cables. The roadway is hanged using steel cables which are connected to two towers and secured by anchors on both ends of the bridge. In addition to the deck slab , the truss system is also featured with truss system just beneath the deck which helps to stiffen and to keeps the deck in precise position to reduce the tendency of the roadway to sway.
When the load is applied to the suspension bridge, the deck slab possess compression and then travels up the ropes, cables or chains to transfer the compression to the towers. The towers then dissipate the compression directly into the earth by anchors. The supporting cables which run parallel to the bridge possess tension forces, and these are connected to anchorages.
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